Light with a wavelength λ of 700 to 1000 nm can pass through the interior of a living body relatively easily compared to light of other wavelengths, and therefore, there have been attempts to use 700 to 1000 nm light to non-invasively measure the fluctuation volumes of hemoglobin, oxygenated hemoglobin, and cytochrome aa3, which have absorption bands of 700 to 1000 nm.
For example, a method (refer, for example, to non-patent literature 1) has been disclosed to obtain the oxygenated hemoglobin fluctuation volume, the reduced hemoglobin fluctuation volume, and the total hemoglobin fluctuation volume based on the light absorbance changes ΔA(780), ΔA(805), and ΔA(830) at various wavelengths λ calculated by transmitting 3 types of light, with wavelengths λ of 780, 805, and 830 nm, into a living body, and receiving the reflected light and transmitted light from within the living body.
With this kind of measurement method, the light absorbance changes ΔA(780), ΔA(805), and ΔA(830) at various wavelengths λ are calculated and the fluctuation volume from a given standard of oxygenated hemoglobin fluctuation volume, etc. is thus obtained. For example, the standard for the measurement of light absorbance changes ΔA(780), ΔA(805), and ΔA(830) is the level of light absorbance, etc. at the time of mounting the optical measuring instrument (for example, an oxygen monitor for a living body, etc.) on the patient (living body), and the level obtained is the amount of change when taking that initial level as the standard. However, no information about the condition of the patient can be obtained at all at the time of mounting the optical measuring instrument on the patient, and therefore, there are the problems that the measurement results from a different day cannot be compared, and that measurement results obtained between different patients cannot be compared.
In order to compare measurement results obtained on a different day or to compare measurement results obtained from different patients, it is necessary to calculate the absolute value A(λ) of light absorbance (called “absolute light absorbance” hereinafter) rather than calculating the light absorbance change ΔA(λ) that takes the level of light absorbance at the time of mounting the optical measuring instrument on the patient as the standard. In order to calculate the absolute value A(λ), the origin of light absorbance must be set.
When the target of measurement is a chemical substance and not a living body such as a patient, there are the well-known methods of taking a fluid that does not contain any components to be measured (for example, distilled water, etc.) as the origin of light absorbance, or using barium sulfate powder as a standard of 100% reflectivity for measuring fabric and printed material. On the other hand, in order to apply this to an optical measuring instrument that irradiates measurement light on one part of a diffusion material such as a living body, which is the measurement target of the present invention, and detects the measurement light that exits from another part, it is desirable that the origin of light absorbance fulfill the conditions of having the same diffusion transmissivity as the living body, of having the same photosensitivity T(λ) as the living body, and also of changing very little over time.
Here, “photosensitivity” T(λ) shall be the value calculated by formula (1) below.T(λ)=I(λ)/Io(λ)  (1)
Io(λ) is the light intensity of the light transmitted, and I(λ) is the light intensity of the light received.
Moreover, the “light absorbance” A(λ) shall be the value calculated by formula (2) below.A(λ)=−log T(λ)  (2)
Thus, because the condition of having the same photosensitivity T(λ) as the living body is fulfilled, a standard device for origin of light absorbance may be offered that can set the origin of light absorbance (for example, refer to patent literature 1). FIG. 5 indicates a perspective view indicating the configuration of a conventional standard device for origin of light absorbance. In addition, FIG. 4 is a diagram for explaining the measurement method using living body oxygen monitor 26.
This kind of conventional standard device for origin of light absorbance 20 comprises a polyacetal resin plate (light diffusion plate) 3 through the interior of which measurement light can be diffused and transmitted, and a neoprene rubber plate 4 affixed to the back surface of the polyacetal resin plate 3.
Meanwhile, the living body oxygen monitor 26 provides a light transmission light guide 6 for transmitting the measurement light, a light receiving light guide 8 for receiving the measurement light, an operating key 28 for operating the living body oxygen monitor 26, a liquid crystal display panel 30, and a recorder 32. Moreover, a light transmitting unit (light transmission terminal) 7 that irradiates light onto the target for measurement (a living body, standard device for origin of light absorbance) is provided on the end of the light transmission light guide 6; and a light receiving unit (light receiving terminal) 9 that receives measurement light from the target for measurement is provided on the end of the light reception light guide 8.
When setting up the origin of the light absorbance using this kind of standard device for origin of light absorbance 20, the light transmitting unit 7 and the light receiving unit 9 come into contact with the surface of the polyacetal resin plate 3, and the light absorbance a(λ) is calculated by using the light receiving unit 9 to detect the measurement light that passes through the interior of the polyacetal resin plate 3. Then, by conducting automatic gain settings, etc. using the living body oxygen monitor 26 main unit, calibration is conducted such that the output of the light absorbance a(λ) is “0” (origin).
As indicated in FIG. 4, when measuring the absolute light absorbance A(λ) of the interior of the brain of the patient using the living body oxygen monitor 26, the light transmitting unit 7 and the light receiving unit 9 come into contact with the surface of the head of the patient, and the change in light absorbance ΔA(λ) from the origin “0” is derived by using the light receiving unit 9 to detect the measurement light that has passed through the brain of the patient. Then, the absolute light absorbance A(λ) is calculated by adding the light absorbance a(λ) to the derived change in light absorbance ΔA(λ).
[Patent literature 1] Japan Unexamined Patent Publication No. H5-277118
[Non-patent literature 1] “Pediatrics” 75, 217-225 (1985), “Artificial Organs” 19, 535-538 (1990).
Nonetheless, as indicated in FIG. 4 for example, when obtaining the absolute light absorbance A(λ) of the brain of the patient, the light transmitting unit 7 and light receiving unit 9 of the living body oxygen monitor contacted the surface of the patients head, but, because contamination such as dandruff or oils and the like on the surface of the head of the patient could adhere to the light transmitting unit 7 and the light receiving unit 9, after the measurement was completed, it was necessary to use a cotton swab or the like immersed in alcohol or the like to remove the contamination adhering to the light transmitting unit 7 and the light receiving unit 9; and this required labor.
In addition, if the contamination adhering to the light transmitting unit 7 and the light receiving unit 9 was not completely removed and the origin of light absorbance was set using the standard device for origin of light absorbance 20, the origin of light absorbance was set while the specified light absorbance still could not be obtained, and errors were included in the measurements as a result.